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He urged people worried that their PIN number might have been compromised to contact their branch.

Parnell man Matt Gers, 35, received a call from the National Bank late on Saturday night asking him if he was in New Zealand.

"They told me money was being withdrawn from my current account in South Africa and they were alerted to it because there had been four $500 withdrawals in quick succession."

His partner, who had used an ASB card at an ANZ machine, received a similar call from her bank yesterday morning to say almost $500 had been taken - leaving the couple to wonder how many others had been affected.

"Who knows how it happened, if it was one particular ATM [or not]," Mr Gers said. "There may well be people who have their PIN numbers skimmed but the ones who have done this haven't tried to use that account yet so they wouldn't be aware of it at all."

Another National Bank customer said they'd lost $3000 from one account.

Mr Gers was told by a bank representative on Saturday night that 150 customers were known to have been affected.

ASB's general manager of cards, transactions and payments, Shaun Drylie, said the bank had been alerted to the issue and was investigating the impact on customers.

The ANZ spokesman said customers who were known to have used the machines were being spoken to. "All our ATMs are fitted with anti-skimming devices. We also have sophisticated fraud protection software providing constant, live reports.

"Unfortunately, this is a game of cat and mouse. As fast as ATM manufacturers and banks come up with ways to prevent this happening, the fraudsters come up with ways to overcome those measures."

Mr Gers said the fraudsters were always one step ahead "to come up with something new".

ANZ said anyone who had lost money as a result of the latest scam would be reimbursed.